Synthesis of Ruthenium Polyhydride Clusters with 1, 4, 7

A novel tetraruthenium octahydrido cluster complex with 1,4,7-triazacyclononane (Cn), [(CnRu)4(μ-H)6(μ3-H)2](BF4)4, was synthesized via spontaneous ...
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Organometallics 2010, 29, 4305–4311 DOI: 10.1021/om100549d

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Synthesis of Ruthenium Polyhydride Clusters with 1,4,7-Triazacyclononane-Type Ligands: Stereo and Electronic Effects of Ancillary Ligands Kyo Namura, Satoshi Kakuta, and Hiroharu Suzuki* Department of Applied Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-8552, Japan Received June 4, 2010

A ruthenium bis(η2-dihydrogen)-hydrido complex with 1,4,7-triazacyclononane (Cn), [CnRuH(H2)2](BPh4) (2a-BPh4), was prepared. The infrared spectra, longitudinal relaxation time (T1) measurements, and density functional theory calculations reveal that the electron density at the metal center of complex 2a is higher than that of the 1,4,7-trimethyl-1,4,7-triazacyclononane (Cn*) analogue [Cn*RuH(H2)2]þ (2b). Complex 2a-BF4 is converted into the tetranuclear octahydrido cluster complex [(CnRu)4(μ-H)6(μ3-H)2](BF4)4 (3-BF4) via spontaneous dehydrogenation, whereas the dehydrogenation of 2b-PF6 exclusively affords the diruthenium trihydrido complex [(Cn*Ru)2(μ-H)3](PF6)2 (4-PF6).

Introduction Transition-metal clusters often exhibit unique reactivity stemming from the multiple coordination of substrates and multielectron transfers.1 We have established synthetic methods for preparing dinuclear,2a trinuclear,2b tetranuclear,2c and pentanuclear2d homo- and heteronuclear polyhydrido clusters with substituted cyclopentadienyl groups that serve as auxiliary ligands and have demonstrated the cooperation between adjacent metal atoms in the substrate activation step through reactions with compounds such as ammonia, hydrazine, silanes, *To whom correspondence should be addressed. E-mail: hiroharu@ n.cc.titech.ac.jp. Tel: (þ81)-3-5734-2148. Fax: (þ81)-3-5734-3913. (1) (a) Adams, R. D., Cotton, F. A., Eds. Catalysis by Di- and Polynuclear Metal Cluster Complexes; Wiley-VCH: New York, 1998. (b) Gates, B. C., Guzei, L., Knozinger, V. H., Eds. Metal Clusters in Catalysis; Elsevier: Amsterdam, 1986. (c) S€uss-Fink, G.; Meister, G. In Advances in Organometallic Chemistry, Vol. 35; Cotton, F. A., Wilkinson, G., Murillo, C. A., Bochmann, M., Eds.; John Wiley & Sons, Inc.: New York, 1999; pp41-134. (d) Deeming, A. J. In Advances in Organometallic Chemistry, Vol. 26; Cotton, F. A., Wilkinson, G., Murillo, C. A., Bochmann, M., Eds.; John Wiley & Sons, Inc.: New York, 1999; pp 1-96. (e) Hogarth, G. In Comprehensive Organometallic Chemistry III, Vol. 6; Crabtree, R. H., Mingos, D. J. P., Eds.; Elsevier: Oxford, UK, 2007; Chapter 6.06. (f ) Wilton-Ely, J. D. In Comprehensive Organometallic Chemistry III, Vol. 6; Crabtree, R. H., Mingos, D. J. P., Eds.; Elsevier: Oxford, UK, 2007; Chapter 6.17. (2) (a) Suzuki, H.; Omori, H.; Lee, D. H.; Yoshida, Y.; Moro-oka, Y. Organometallics 1988, 7, 2243. (b) Suzuki, H.; Kakigano, T.; Tada, K.; Igarashi, M.; Matsubara, K.; Inagaki, A.; Oshima, M.; Takao, T. Bull. Chem. Soc. Jpn. 2005, 78, 67. (c) Ohki, Y.; Uehara, N.; Suzuki, H. Angew. Chem., Int. Ed. 2002, 41, 4085. (d) Ohki, Y.; Uehara, N.; Suzuki, H. Organometallics 2003, 22, 59. (3) (a) Nakajima, Y.; Kameo, H.; Suzuki, H. Angew. Chem., Int. Ed. 2006, 45, 950. (b) Takao, T.; Moriya, M.; Suzuki, H. Organometallics 2007, 26, 1349. (c) Kawashima, T.; Takao, T.; Suzuki, H. J. Am. Chem. Soc. 2007, 129, 11006. (d) Moriya, M.; Tahara, A.; Takao, T.; Suzuki, H. Eur. J. Inorg. Chem. 2009, 23, 3393. (4) (a) Shima, T.; Suzuki, H. Organometallics 2000, 19, 2420. (b) Ito, J.; Shima, T.; Suzuki, H. Organometallics 2006, 25, 1333. (c) Kameo, H.; Nakajima, Y.; Suzuki, H. Angew. Chem., Int. Ed. 2008, 47, 10159. (d) Shima, T.; Sugimura, Y.; Suzuki, H. Organometallics 2009, 28, 871. r 2010 American Chemical Society

and various hydrocarbons.3,4 Moreover, the reversible dihydrogen uptake by polyhydrido complexes has been exhibited by several polyhydrido clusters and has recently attracted the interest of many researchers.5 The reactivity of cluster complexes mostly depends on the electronic nature of the ancillary ligands. Therefore, the possibility of synthesizing new polyhydrido complexes having noncyclopentadienyl ligands should be examined. Recently, we reported the synthesis of a novel ruthenium bis(η 2 dihydrogen)-hydrido complex bearing 1,4,7-trimethyl-1,4,7triazacyclononane (Cn*), [Cn*RuH(H2)2]þ (2b), which efficiently served as a precursor for two unprecedented heterometallic dinuclear complexes, [Cn*Ru(μ-H)3IrCp*]þ and [Cn*Ru(μ-H)3Os(H)Cp*]þ.6 To the best of our knowledge, only Wieghardt et al. have reported examples of dinuclear polyhydrido complexes of rhodium and iron having 1,4,7-triazacyclononane-type ligands,7 but the reactivity of these complexes remains unclear. In this paper, we report the synthesis of a bis(η2-dihydrogen)hydrido complex with a 1,4,7-triazacyclononane (Cn) ligand [CnRuH(H2)2]þ (2a), and we compare the electronic properties of 2a with those of the Cn* analogue 2b, whose synthesis has been reported recently.6 The results of this study revealed that the Cn ligand is more electron donating than the Cn* ligand, although the hydrogen atom is less electron donating than the methyl group. Furthermore, complex 2a spontaneously tetramerized and afforded an unprecedented tetraruthenium octahydrido (5) (a) Brayshaw, S. K.; Ingleson, M. J.; Green, J. C.; McIndoe, J. S.; Raithby, P. R.; Kociok-K€ ohn, G.; Weller, A. S. J. Am. Chem. Soc. 2006, 128, 6247. (b) Adams, R. D.; Captain, B.; Beddie, C.; Hall, M. B. J. Am. Chem. Soc. 2007, 129, 986. (c) Adams, R. D.; Captain, B.; Smith, M. D.; Beddie, C.; Hall, M. B. J. Am. Chem. Soc. 2007, 129, 5981. (6) Shima, T.; Namura, K.; Kameo, H.; Suzuki, H. Organometallics 2010, 29, 337. (7) Hanke, D.; Wieghardt, K.; Nuber, B.; Lu, R. S.; McMullan, R. K.; Koetzle, T. F.; Bau, R. Inorg. Chem. 1993, 32, 4300. Published on Web 09/09/2010

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cluster. On the other hand, complex 2b afforded a diruthenium trihydrido complex, and the bulkiness of the ancillary ligands affected the nuclearity of the polyhydrido complexes.

Results and Discussion Synthesis of Ruthenium Bis(η2-dihydrogen)-hydrido Complex [CnRuH(H2)2]X (2a-X; X = BPh4, BF4, PF6). A ruthenium bis(η2-dihydrogen)-hydrido complex [CnRuH(H2)2]X (2a-X; X = BPh4, BF4, PF6) was prepared according to a previously reported method using CnRuCl38 as a precursor.6 The reaction of NaBH4 with CnRuCl3 (1) in a mixed solvent of ethanol and dichloromethane at ambient temperature smoothly yielded 2a-BH4. The addition of NaBPh4 to 2a-BH4 in ethanol and washing the precipitates with methanol afforded the tetraphenylborate salt 2a-BPh4 in 63% yield as a pale yellow solid (eq 1). Although the anion exchange of 2a-BH4 with NH4BF4 and NH4PF6 smoothly proceeded to yield the corresponding salts, 2a-BF4 and 2a-PF6, respectively, they could not be separated from the salts formed during the reaction.

Compound 2a was characterized by 1H and 13C NMR spectroscopy, X-ray diffraction study, and elemental analysis. The 1H NMR spectrum of 2a exhibited a sharp hydrido signal (5H) at δ -12.31 in acetonitrile-d3, whereas the hydrido signal for the Cn* analogue 2b-PF6 appeared at δ -12.53 in acetone-d6. Although the hydrido ligands, namely, the terminal hydride and two η2-H2 ligands, are magnetically nonequivalent, they were observed to be equivalent as a singlet in the temperature range -115 to 25 °C. This is most certainly due to the presence of a rapid site-exchange process among these hydrido ligands.9 The molecular structure of 2a-BPh4 is illustrated in Figure 1 with the relevant bond lengths and angles. Although the hydrido ligand and the coordinated dihydrogen molecules could not be located in the differential maps, their locations were reliably estimated because 2a-BPh4 possessed two short Ru-N bonds (2.121(4), 2.143(5) A˚) and one long Ru-N bond (2.217(5) A˚), which were also observed in the Cn* analogue 2b-PF6.6 Density functional theory (DFT, B3PW91 level)10 calculations were performed for 2a, and the resulting optimized structure of 2a is shown in Figure 2 with that of 2b. The structural trends estimated for 2a resemble those calculated for 2b. According to the calculations, the hydrido ligand occupied the trans position of the long Ru-N bond, whereas the two dihydrogen ligands occupied the trans positions of the short Ru-N bonds. Furthermore, the calculations indicated that the average H-H distance of the coordinated dihydrogen in 2a (0.9463 A˚) is longer than that in 2b (0.9368 A˚) by 0.01 A˚. An increase in the H-H distance would reflect an enhancement of the back-donation from the metal atom to the dihydrogen ligand in 2a. Therefore, the electron density at the metal atom in 2a could be higher than that in (8) Wieghart, K.; Herrmann, W.; Koeppen, M.; Jibril, I.; Huttner, G. Z. Naturforsch., B 1984, 39, 1335. (9) Kubas, G. J. In Metal Dihydrogen and σ-Bond Complexes; Fackler, J. P., Ed.; Kluwer Academic/Plenum Publishers: New York, 2001. (10) (a) Becke, A. D. Phys. Rev. 1988, A38, 3098. (b) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (c) Perdew, J. P.; Wang, Y. Phys. Rev. 1992, B45, 13244.

Figure 1. ORTEP drawing of 2a-BPh4 with thermal ellipsoids at the 30% probability level. Selected bond lengths (A˚) and angles (deg): Ru1-N1, 2.143(5); Ru1-N2, 2.121(4); Ru1-N3, 2.217(5); N1-Ru1-N2, 79.97(18); N1-Ru1-N3, 79.75(18); N2-Ru1-N3, 79.50(17).

Figure 2. Optimized structures of 2aþ (left) and 2bþ (right) at the B3PW91 level. Hydrogen atoms on carbons are omitted for clarity. Natural charges are underlined (values shown for methyl groups are the total charges of carbon and bonding hydrogen atoms.).

2b. The infrared spectra of 2a-BPh4 and 2b-BPh4 showed peaks that could be assigned to the stretching vibrations of Ru-H at 1971 and 1946 cm-1 for 2a and 2b, respectively. In these spectra, the stretching vibration of the coordinated dihydrogen atoms in 2a and 2b appeared at 2331 and 2344 cm-1, respectively (Figure 3).11 These values are very similar to those reported for [Tp*RuH(H2)2] (νRu-H: 1950 cm-1, νH-H: 2361 cm-1, Tp*: hydridotris(3,5-dimethylpyrazolyl)borate).12 Furthermore, the absorption peak attributed to the stretching vibration of the dihydrogen ligand of 2a shifted slightly to a lower frequency in comparison to that of 2b, implying a reduction in the H-H bond order in 2a. These results are quite consistent with the tendencies observed in the longitudinal relaxation times (T1) for these complexes. Although the T1 measurement was not performed below -115 °C because of the high viscosity of the solvent, the T1 value for 2a was larger than that for 2b in the temperature range -115 to -60 °C. The minimum T1 values were estimated at 42 and 18 ms at -115 °C for 2a-BPh4 and 2b-PF6, respectively. To estimate the charge distributions in 2a and 2b, we carried out natural population analysis. 13 The electron (11) The absorption bands were assigned from infrared spectra of 2bPF6 and its isotopomers. Details are given in the Supporting Information. (12) Moreno, B.; Sabo-Etienne, S.; Chaudret, B.; Rodriguez, A.; Jalon, F.; Trofimenko, S. J. Am. Chem. Soc. 1995, 117, 7441. (13) Reed, A. E.; Weinstock, R. B.; Weinhold, F. J. Chem. Phys. 1985, 83, 735.

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Figure 3. Infrared spectra (KBr) of (a) 2a-BPh4 and (b) 2b-BPh4.

density at the metal atom in 2a is significantly higher than that in 2b. The polarization of the N-substituent, N-C(methyl), or N-H bond in the triamine ligand crucially affects the electron density at the metal center. The hydrogen atom attached to the nitrogen in the Cn ligand is more positively charged than the methyl group in the Cn* ligand. Therefore, the nitrogen atom in the Cn ligand is more negatively charged than that in the Cn* ligand. As a result, the electron density at the metal center in 2a increases in comparison to that in 2b. To the best of our knowledge, complex 2a is the first reported example of a ruthenium bis(η2-dihydrogen) complex ligated with a Cn ligand. Thus far, related ruthenium bis(η2-dihydrogen) complexes containing bulky phosphine ligands14 or hydridotris(pyrazolyl) borate ligands8 have been reported in addition to the Cn* analogue 2b. Synthesis of Tetracationic Tetraruthenium Cluster [(CnRu)4(μ-H)6(μ3-H)2](BF4)4 (3-BF4). At room temperature, an acetone solution of 2a-BF4 gradually turned dark red with the liberation of dihydrogen to form a tetracationic tetraruthenium cluster, [(CnRu)4(μ-H)6(μ3-H)2](BF4)4 (3-BF4) (eq 2). The red crystals of 3-BF4 that precipitated from the solution were isolated in 29% yield based on complex 1. Complex 3 was not obtained from 2a-BPh4 because of the low solubility of 2a-BPh4 in acetone. Complex 3-BF4 is an electron-precise, 60-electron complex. In the solid state, 3-BF4 is sufficiently stable in air, although its solution reacts with oxygen to produce unidentified complexes.

The 1H NMR spectrum of 3-BF4 showed two sharp signals for the hydrido ligands at δ -19.96 (7H) and δ -20.21 (1H) as well as a broad singlet stemming from N-H at δ 5.49 (14) (a) Grellier, M.; Vendier, L.; Chaudret, B.; Albinati, A.; Rizzato, S.; Mason, S.; Sabo-Etienne, S. J. Am. Chem. Soc. 2005, 127, 17592. (b) Abdur-Rashid, K.; Gusev, D. G.; Lough, A. J.; Morris, R. H. Organometallics 2000, 19, 1652. (c) Borowski, A. F.; Donnadieu, B.; Daran, J.-C.; Sabo-Etienne, S.; Chaudret, B. Chem. Commun. (Cambridge, U.K.) 2000, 543.

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(12H) in acetonitrile-d3. Complex 3-BF4 underwent a H/D exchange reaction with acetonitrile-d3 at 70 °C (Figure 4). In this reaction, the formation of isotopomer 3-dn (n = 1-7) was observed, which indicates that 3-BF4 is an octahydrido complex.15 Furthermore, X-ray diffraction study of 3-BF4 unambiguously demonstrated a tetranuclear structure (Figure 5). The six Ru-Ru bond lengths were nearly equal (2.9515(5)2.9886(5) A˚), and the Ru4 core was pseudotetrahedral. The six hydrido ligands bridged each of the Ru-Ru bonds. Although the remaining two hydrido ligands were not located in the differential maps, each of them was proposed to triply bridge the Ru atoms. DFT calculations performed on the tetranuclear complex 3 supported the proposed location of the hydrido ligands. The distances between the adjacent two metal-bound hydrogen atoms range from 1.7176 to 1.7749 A˚, suggesting that complex 3 is a classical metal hydride (Figure 6).16 This is in contrast with the related 60-electron hexahydrido cluster [H6Ru4(C6H6)4]2þ, which possesses nonclassical hydrido ligands.17 As shown in Figure 6, the hydrido ligands are classified into four groups: (H3), (H2, H4, H5, and H6), (H7 and H8), and (H1). The first two groups are positioned adjacent to the triply bridging face-capping hydrides, H7 and H8. These seven hydrides are assumed to mutually exchange the coordination sites. As a result, the 1H signal for the hydrido ligands is separated into two groups with the intensities of 1H and 7H. The site-exchange process is so fast that the line shape of the hydride signal does not change in the temperature range -80 to 25 °C. The T1 values for the hydrides in 3-BF4 are small (154 ms (δ -19.96, 7H) and 142 ms (δ -20.21, 1H) at -80 °C in acetone-d6/acetonitrile-d3, 2:1). Nevertheless, these T1 values are still larger than the criteria for dihydrogen complexes. However, the relatively small T1 values mentioned above suggest that the site-exchange process occurs via an intermediary dihydrogen species. Condensation of the mononuclear dihydrogen complex 2aBF4 afforded the tetranuclear complex 3-BF4. To the best of our knowledge, complex 3-BF4 is the first example of a tetranuclear complex containing both Cn and hydrido ligands, although tetranuclear complexes with bridging oxo,18 hydroxo,19 and hydroborate20 ligands are already known. In addition, the syntheses of rhodium polyhydrido clusters [Rh6(PR3)6H12]2þ (R = iPr, Cy) by the dehydrogenation of mononuclear dihydrogen complexes have been reported by Weller et al.5a,21 Synthesis of Dicationic Diruthenium Trihydrido Complex [(Cn*Ru)2 (μ-H)3 ](PF 6 )2 (4-PF 6 ). Unlike 2a, complex 2b, which has a sterically demanding Cn* ligand, dimerized with (15) The H/D exchange reaction of polyhydride complexes supplies useful information for quantifying the hydride ligands. See refs 2b and 17a. (16) Three initial structures, [(CnRu)4 (μ-H)6 (μ 3 -H)2 ]4þ (3A), [(CnRu)4(μ-H)5(μ3-H)3]4þ (3B), and [(CnRu)4(μ-H)4(μ3-H)4]4þ (3C), were optimized at the B3PW91 level and converged to the same optimized structure similar to 3A. Details are given in the Supporting Information. (17) (a) S€ uss-fink, G.; Plasseraud, L.; Maisse-Franc-ois, A.; StoeckliEvans, H.; Berke, H.; Fox, T.; Gautier, R.; Saillard, J.-Y. J. Organomet. Chem. 2000, 609, 196. (b) Meister, G.; Rheinwald, G.; Stoeckli-Evans, H.; S€uss-fink, G. J. Chem. Soc., Dalton Trans. 1994, 22, 3215. (18) (a) Wieghardt, K.; Bossek, U.; Gebert, W. Angew. Chem., Int. Ed. 1983, 22, 328. (b) Wieghardt, K.; Ventur, D.; Tsai, Y. H.; Krueger, C. Inorg. Chim. Acta 1985, 99, L25. (19) (a) Drueke, S.; Wieghardt, K.; Nuber, B.; Weiss, J.; Bominaar, E. L.; Sawaryn, A.; Winkler, H.; Trautwein, A. X. Inorg. Chem. 1989, 28, 4477. (b) Wieghardt, K.; Kleine-Boymann, M.; Nuber, B.; Weiss, J. Inorg. Chem. 1986, 25, 1654. (20) Giese, H.-H.; Habereder, T.; N€ oth, H.; Ponikwar, W.; Thomas, S.; Warchhold, M. Inorg. Chem. 1999, 38, 4188. (21) Ingleson, M. J.; Mahon, M. F.; Raithby, P. R.; Weller, A. S. J. Am. Chem. Soc. 2004, 126, 4784.

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Figure 4. H/D exchange reaction of 3-BF4 with acetonitrile-d3 showing hydride resonances (room temperature, acetonitrile-d3): (a) initial spectrum; (b) after 5 h; (c) after 10 h; (d) after 15 h; (e) after 20 h; and (f) after 25 h.

Figure 5. ORTEP drawings of the cationic part of 3-BF4 (left) and the Ru4 core (right) with thermal ellipsoids at the 30% probability level. Selected bond lengths (A˚): Ru1-Ru2, 2.9647(5); Ru1-Ru3, 2.9603(5); Ru1-Ru4, 2.9709(5); Ru2-Ru3, 2.9515(5); Ru2-Ru4, 2.9606(5); Ru3-Ru4, 2.9886(5); Ru1-N1, 2.139(4); Ru1-N2, 2.132(4); Ru1-N3, 2.129(4).

diruthenium complex [(Cn*Ru)2(μ-H)3](PF6 )2 (4-PF6 ) in 55% yield (eq 3). The addition of NaBPh4 to the acetone solution of 4-PF6 resulted in 4-BPh4, quantitatively.

Figure 6. Optimized structure of 34þ at the B3PW91 level. Cn ligands are omitted for clarity. Selected bond lengths (A˚ ): Ru1-Ru2, 2.9915; Ru1-Ru3, 3.0153; Ru1-Ru4, 3.0204; Ru2-Ru3, 3.0175; Ru2-Ru4, 3.0176; Ru3-Ru4, 3.0802; H2-H7, 1.7176; H3-H7, 1.7742; H6-H7, 1.7272; H3-H8, 1.7749; H4-H8, 1.7210; H5-H8, 1.7216.

the liberation of hydrido ligands to generate a dicationic diruthenium trihydride, 4. When a solution of 2b-PF6 in acetone was stirred for 102 h in the presence of a catalytic amount of palladium charcoal, the color of the solution turned purple. Subsequent purification of this solution by column chromatography on alumina afforded the dicationic

The 1H NMR spectrum of 4-PF6 was paramagnetically shifted and exhibited three broad signals corresponding to Cn* at δ 18.94 (18H, w1/2 = 239.7 Hz), 6.25 (12H, w1/2 = 132.0 Hz), and -5.81 (12H, w1/2 = 97.5 Hz) ppm. Signals corresponding to hydrido ligands were not observed. X-ray diffraction study of 4-BPh4 confirmed the dinuclear structure of 4 (Figure 7). Although all the bridging hydrogen atoms were not located in the difference Fourier synthesis maps, DFT calculations suggested that 4 was a trihydrido complex.22 Furthermore, the (22) Two initial structures, [(Cn*Ru)2(μ-H)3]2þ (4A) and [(Cn*Ru)2(μ-H)5]2þ (4B), were optimized at the B3PW91 level, and only 4A reproduced the molecular structure of 4-BPh4. Details are given in the Supporting Information.

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Figure 7. ORTEP drawing of the cationic part of 4-BPh4 with thermal ellipsoids at the 30% probability level. Selected bond lengths (A˚): Ru1-Ru2, 2.3778(2); Ru1-N1, 2.1361(17); Ru1N2, 2.1323(16); Ru1-N3, 2.1243(17); Ru2-N4, 2.1270(17); Ru2-N5, 2.1273(16); Ru2-N6, 2.1156(17). The third metalbound hydrogen atom was not located in the difference Fourier synthesis maps.

ORTEP drawing shows that the nitrogen atoms of the Cn* ligand and the bridging hydrido ligands occupy a common facial position in the octahedral geometry. The bulkiness of the ancillary ligand of a mononuclear building block significantly affects the nuclearity of the formed cluster. The Cn-ligated complex 2a and the Cn*ligated complex 2b afford tetranuclear 3 and dinuclear 4, respectively. This is a close analogy for a ruthenium-benzene or hexamethylbenzene system. The hydrogenation of a benzene complex affords a tetraruthenium hexahydrido complex, whereas a bulkier hexamethylbenzene complex affords diruthenium trihydride.23 Despite being an electron-deficient 29-electron complex, 4 is fairly stable in air in the solid state. However, the acetone solution of 4-PF6 reacted with oxygen; as a result, the color of the solution turned blue. Several [(LRu)2(μ-H)n]-type diruthenium complexes such as [(Cp*Ru)2(μ-H)4],2a [(C6Me6Ru)2(μ-H)3]þ,24 [{(Me3P)3Ru}2(μ-H)3]þ,25 and [(Me3P)3Ru(μ-H)3RuC 6Me 6]þ26 have previously been reported. All of these complexes possess 30 electrons; among these, there are no precedents of 29-electron complexes. However, highly σ-donating Cn* ligands are assumed to stabilize electron-deficient metal atoms, and the bulky Cn* ligands would effectively shield the metal atoms from various types of substrates. Consequently, a 29-electron species can be isolated.

Conclusion We have synthesized a cationic bis(η2-dihydrogen)-hydrido complex of ruthenium, [CnRuH(H2)2]þ (2a), by treating CnRuCl3 with NaBH4 in a mixed solvent of ethanol and dichloromethane. The infrared spectra of 2a and its Cn* analogue 2b showed peaks that could be assigned to the stretching vibration of the dihydrogen ligand (νH-H 2a: 2331 cm-1, 2b: 2344 cm-1), and the values of the longitudinal (23) S€ uss-Fink, G.; Therrien, B. Organometallics 2007, 26, 766. (24) (a) Bennett, M. A.; Ennett, J. A. Inorg. Chim. Acta 1992, 198-200, 583. (b) S€ uss-Fink, G.; Therrien, B. Organometallics 2007, 26, 766. (25) Jones, R. A.; Wilkinson, G.; Colquhoun, I. J.; McFarlane, W.; Galas, A. M. R.; Hursthouse, M. B. J. Chem. Soc., Dalton Trans. 1980, 12, 2480. (26) Tschan, M. J.-L.; Cherioux, F.; Therrien, B.; S€ uss-Fink, G. Eur. J. Inorg. Chem. 2007, 4, 509.

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relaxation time (T1) for 2a and 2b were estimated at 42 and 18 ms, respectively, at -115 °C. These results implied that the charge density at the ruthenium atom in 2a was higher than that in 2b, although the electron-releasing effect of hydrogen was weaker as compared to that of the methyl group. This is assumed to be a result of the highly polarized N-H bond of the Cn ligand. The bulkiness of the ancillary ligand in a mononuclear complex applied to cluster synthesis crucially affects the nuclearity of the synthesized cluster. Accordingly, 2a tetramerizes spontaneously to selectively form 3. On the other hand, the Cn* analogue 2b yields a dinuclear paramagnetic trihydrido complex 4 as a result of the mutual steric repulsion of the methyl groups at the nitrogen atoms. The steric shielding effect of the methyl groups probably stabilizes 4 and enables the isolation of the paramagnetic species. The reactivity of these complexes is currently being investigated, and the results will be reported in due course.

Experimental Section General Procedures. The compounds described below were handled under an argon atmosphere, and air and water were completely removed using Schlenk techniques. CnRuCl38 (1) and [Cn*RuH(H2)2](X) (2b-X; X = PF6, BPh4)6 were prepared as previously described. THF-d8, toluene-d8, and diethyl ether were dried over sodium benzophenone ketyl and distilled under an argon atmosphere. The other reagents used in this study were purchased from commercial sources and used without further purification. 1H and 13C NMR spectra were recorded on Varian INOVA 400 and Varian 400-MR Fourier transform spectrometers using tetramethylsilane as the internal standard. The chemical shifts (δ) were reported in parts per million (ppm). Infrared spectra were recorded on a JASCO FT/IR-4200 spectrometer. Elemental analyses were recorded on a PerkinElmer 2400II. ESI-MS spectra were recorded on a JEOL JMST100CS spectrometer. Magnetic susceptibility was measured on a Sherwood Scientific MSB-AUTO at ambient temperature. The diamagnetic correction was estimated from Pascal’s constants. [CnRuH(H2)2](BPh4) (2a-BPh4). A 50 mL Schlenk tube was filled with 1 (146.9 mg, 436.4 μmol), ethanol/dichloromethane mixed solvent (28 mL, 2:1), and NaBH4 (167.3 mg, 4.421 mmol). This solution was stirred for 15 h at room temperature. The color of the solution changed from orange to pale red. The solution was filtered through a glass filter, and the solvent was removed under reduced pressure. The obtained pale orange solid was dissolved in ethanol (20 mL), and NH4PF6 (809.5 mg, 4.966 mmol) was added to it. The resulting solution was stirred until the generation of gas stopped and red precipitates were afforded. Removal of the precipitates by short columns packed with Celite afforded a pale yellow solution of 2a-PF6 and salts. To this solution was added NaBPh4 (704.9 mg, 2.060 mmol), and yellowish-white precipitates were formed. Washing the precipitates with methanol afforded 2a-BPh4 (153.1 mg, 276.1 μmol, 63% yield) as a yellowish-white solid. Single crystals of 2a-BPh4 that were suitable for X-ray analysis were obtained from acetonitrile at -30 °C. The addition of NH4BF4 instead of NH4PF6 afforded 2a-BF4, although 2a-PF6 and 2a-BF4 were not isolated because of the difficulty in separating them from the byproduct salts. Data for 2a-BPh4 are as follows. 1H NMR (400 MHz, acetonitrile-d3, room temperature): δ 7.27 (m, 8H, o-Ph), 6.99 (dd, JHH = 7.1 Hz, 8H, m-Ph), 6.84 (t, JHH = 7.1 Hz, 4H, p-Ph), 5.49 (brs, w1/2 = 32.6 Hz, 3H, NH), 2.97-3.08 (m, 6H, -NCHH-HHCN-), 2.60-2.70 (m, 6H, -NCHH-HHCN-), -12.31 (s, 5H, RuH). T1 (THF-d8/toluene-d8, 5:1): 167 (-40 °C), 81 (-80 °C), 57 (-100 °C), 46 (-110 °C), 42 (-115 °C) ms.

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C NMR (100 MHz, acetonitrile-d3, room temperature): δ 164.7 (q, JCB = 48.9 Hz, ipso-Ph), 136.6 (dt, JCH = 151.2, 7.1 Hz, m-Ph), 126.5 (d, JCH = 150.9 Hz, o-Ph), 122.7 (dt, JCH = 156.3, 7.9 Hz, p-Ph), 50.7 (t, JCH = 138.8 Hz, NCH2). Anal. Calcd for C30H40N3B1Ru1: C, 64.98; H, 7.27; N, 7.58. Found: C, 65.06; H, 7.15; N, 7.71. IR (KBr): 3282 (νN-H), 2331 (νH-H), 1971 (νRu-H) cm-1. Data for 2a-BF4 are as follows. 1H NMR (400 MHz, methanold 4 , room temperature): δ 6.59 (brs, w 1/2 = 20.5 Hz, 3H, NH), 3.06-3.14 (m, 6H, -NCHH-HHCN-), 2.74-2.82 (m, 6H, -NCHH-HHCN-), -12.18 (s, 5H, RuH). 13C NMR (100 MHz, methanol-d4, room temperature): δ 51.3 (t, JCH = 136.4 Hz, NCH2). Data for 2a-PF6 are as follows. 1H NMR (400 MHz, acetoned6, room temperature): δ 6.39 (brs, w1/2 = 24.6 Hz, 3H, NH), 3.24-3.34 (m, 6H, -NCHH-HHCN-), 2.94-3.02 (m, 6H, -NCHH-HHCN-), -12.14 (s, 5H, RuH). 13C NMR (100 MHz, methanol-d4, room temperature): δ 51.2 (t, JCH = 135.7 Hz, NCH2). [(CnRu)4(μ-H)6(μ3-H)2](BF4)4 3 1.5(C3H6O) (3-BF4). A 100 mL Schlenk tube was filled with 1 (506.8 mg, 1.506 mmol), an ethanol/ dichloromethane mixed solvent (70 mL, 2:1), and NaBH4 (573.4 mg, 4.421 mmol). This mixture was stirred for 15 h at room temperature. The solid obtained after the subsequent removal of the solvent was dissolved in acetonitrile (60 mL) and filtered through a glass filter. The filtrate was condensed to 5 mL, and diethyl ether (20 mL) was added to it, affording pale orange precipitates. These precipitates were washed with diethyl ether and dissolved in ethanol (40 mL). To this solution was added NH4BF4 (1.799 g, 17.16 mmol), and red precipitates were afforded. The solution was stirred until the generation of gas stopped, and it was filtered through a glass filter so as to remove the precipitates and excess NH4BF4. The filtrate was condensed to 5 mL, and diethyl ether (25 mL) was added to it, affording yellowish-white precipitates. The precipitates were washed with diethyl ether and dried under reduced pressure. The obtained mixture of 2a-BF4 and byproduct salts was dissolved in acetone (8.0 mL), and the resulting solution was left without stirring at room temperature. After 1 h, the solution turned dark red and red platelet crystals precipitated. Washing the crystals with methanol afforded 3-BF4 (148.3 mg, 108.8 μmol, 29% yield). The crystals contained 1.5 acetone molecules as crystallization solvent. This was confirmed by 1H NMR spectroscopy and X-ray diffraction study. Data for 3-BF4 are as follows. 1H NMR (400 MHz, acetonitrile-d3, room temperature): δ 5.75 (brs, w1/2 = 15.6 Hz, 12H, NH), 3.00-3.12 (m, 24H, -NCHH-HHCN-), 2.85-2.97 (m, 24H, -NCHH-HHCN-), -19.96 (s, 7H, RuH), -20.21 (s, 1H, RuH). T1 (acetone-d6/acetonitrile-d3, 2:1, 7H/1H): 694/653 (20 °C), 281/269 (-40 °C), 202/191 (-60 °C), 154/142 (-80 °C) ms. 13C NMR (100 MHz, acetonitrile-d3, room temperature): δ 52.4 (t, JCH = 138.1 Hz, NCH2). ESI-MS (m/z): 1189 [C24H68N12Ru4 þ (BF4)3]þ. Anal. Calcd for C28.5H77B4F16N12O1.5Ru4: C, 25.10; H, 5.69; N, 12.33. Found: C, 25.30; H, 5.49; N, 11.98. H/D Exchange Reaction of 3-BF4 with Acetonitrile-d3. A Roto Tite NMR sample tube was charged with 3-BF4 (4.3 mg, 3.2 μmol) and acetonitrile-d3 (0.40 mL). The reaction was carried out at 70 °C and monitored by 1H NMR spectroscopy. [(Cn*Ru)2(μ-H)3](PF6)2 (4-PF6). A 50 mL Schlenk tube was filled with 2b-PF6 (305.0 mg, 722.1 μmol), Pd/C (10 wt %, 27.8 mg, 26.1 μmol), and acetone (6.0 mL). The mixture was stirred for 102 h at room temperature. The color of the solution changed to purple. After the Pd/C was removed using short columns packed with Celite, the filtrate was condensed to 5 mL. The addition of diethyl ether (30 mL) afforded purple precipitates. The precipitates were washed with THF and purified by column chromatography on alumina with acetone. Removal of the solvent under reduced pressure afforded 4-PF6 (165.3 mg, 197.3 μmol, 55% yield) as a purple solid. Data for 4-PF6 are as follows. 1H NMR (400 MHz, acetoned6, room temperature): δ 18.94 (brs, w1/2 = 239.7 Hz, 18H, 13

Namura et al. NCH3), 6.25 (brs, w1/2 = 132.0 Hz, 12H, -NCHH-HHCN-), -5.81 (brs, w1/2 = 97.5 Hz, 12H, -NCHH-HHCN-). ESI-MS (m/z): 692 [C18H45N6Ru2 þ PF6]þ. μeff = 1.8 μB. Anal. Calcd for C18H45N6P2F12Ru2: C, 25.81; H, 5.41; N, 10.03. Found: C, 25.65; H, 5.19; N, 9.91. [(Cn*Ru)2(μ-H)3](BPh4)2 (4-BPh4). A 50 mL Schlenk tube was filled with 4-PF6 (110.8 mg, 132.3 μmol) and acetone (1.8 mL). The addition of NaBPh4 (301.1 mg, 879.9 μmol) afforded purple precipitates. Washing the precipitates with methanol afforded 4-BPh4 (154.7 mg, 130.4 μmol, 99% yield). Single crystals suitable for X-ray analysis were obtained from DMSO/diethyl ether at room temperature. Computational Details. Density functional theory calculations were performed at the B3PW91 level10 in conjunction with the Stuttgart/Dresden ECP27 and associated with triple-ζ SDD basis sets for Ru. For N and H, 6-311G(d,p) basis sets were employed; for C, basis set 6-31G(d) was used. All calculations were performed utilizing the Gaussian 03 program. 28 The molecular structures were drawn by using the GaussView, version 4.1.2, program.29 Frequency calculations were performed at the same level of theory at which the geometry optimizations were performed on optimized structures to ensure that the minima exhibited only positive frequencies. Information on the atom coordinates (xyz files) of all the optimized structures is presented in the Supporting Information. X-ray Data Collection and Reduction. Single crystals of 2aBPh4, 3-BF4, and 4-BPh4 suitable for X-ray analyses were obtained from the preparations described above and mounted on nylon Cryoloops with Paratone-N (Hampton Research Corp.). The X-ray diffraction experiments on 2a-BPh4, 3-BF4, and 4-BPh4 were performed using a Rigaku R-AXIS RAPID imaging plate diffractometer with a graphite-monochromated Mo KR radiation source (λ = 0.71069 A˚). Cell refinement and data reduction were carried out using the PROCESS-AUTO program.30 The intensity data were corrected for Lorentzpolarization effects and empirical absorption. The structures of 2a-BPh4, 3-BF4, and 4-BPh4 were determined by direct methods using the SHELX-97 program.31 All non-hydrogen atoms were found by a difference Fourier synthesis. The refinement was carried out using the least-squares methods based on F2 with all measured reflection data. All non-hydrogen atoms, except one BF4 anion and the acetone included in 3-BF4, were refined anisotropically. All of the hydrogen atoms, except those bonded to metals, were included in the calculated positions and refined using a riding model. The metal-bound hydrogen atoms (27) Andrae, D.; Haussermann, U.; Dolg, M.; Stoll, H.; Preuss, H. Theor. Chim. Acta 1990, 77, 123. (28) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.;Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, Revision D.02; Gaussian, Inc.: Wallingford, CT, 2004. (29) Dennington, R., II.; Keith, T.; Millam, J. GaussView, Version 4.1; Semichem, Inc.: Shawnee Mission, KS, 2007. (30) PROCESS-AUTO, Automatic Data Acquisition and Processing Package for Imaging Plate Diffractometer; Rigaku Corporation: Tokyo, Japan, 1998. (31) Sheldrick, G. M. SHELX-97, Program for Crystal Structure Determination; University of G€ottingen: G€ottingen, Germany, 1997.

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Table 1. Crystallographic Data of 2a-BPh4, 3-BF4, and 4-BPh4

empirical formula fw cryst description cryst color cryst size (mm) crystallizing solution cryst syst space group a (A˚) b (A˚) c (A˚) β (deg) volume (A˚3) Z value Dcalc (g/cm3) measurement temp (°C) μ(Mo KR) (mm-1) diffractometer radiation monochromator 2θmax (deg) reflns collected indep reflns reflns obsd (>2σ) abs corr type abs transmn R1 (I > 2σ(I)) wR2 (I > 2σ(I)) R1 (all data) wR2 (all data) data/restraints/params goodness of fit on F2 largest diff peak and hole (e A˚-3)

2a-BPh4

3-BF4

4-BPh4

C30H40B1N3Ru1 554.53 needle colorless 0.71  0.09  0.07 acetonitrile (-30 °C) monoclinic P21 (# 4) 9.7101(16) 10.2653(19) 13.582(2) 92.650(5) 1352.3(4) 2 1.362 -120 0.603 Rigaku R-AXIS RAPID Mo KR (λ = 0.71069 A˚) graphite 55 10 212 3199 (Rint = 0.0478) 5120 empirical 0.5334 (min.) 1.0000 (max.) 0.0467 0.1157 0.0566 0.1290 5808/1/316 1.119 1.396 and -1.049

C24H68B4F16N12Ru4 3 (C3H6O1)1.5 1363.54 platelet red 0.46  0.30  0.06 acetone (25 °C) monoclinic P21/n (# 14) 11.9546(5) 21.0770(8) 20.2021(8) 93.9900(13) 5077.9(4) 4 1.784 -100 1.266 Rigaku R-AXIS RAPID Mo KR (λ = 0.71069 A˚) graphite 55 38 215 11 701 (Rint = 0.0523) 9638 empirical 0.7250 (min.) 1.0000 (max.) 0.0488 0.1303 0.0578 0.1376 11 360/6/588 1.038 1.591 and -1.463

C66H85B2N6Ru2 1186.16 platelet purple 0.41  0.13  0.03 DMSO/Et2O (25 °C) monoclinic P21/n (# 14) 12.6657(3) 35.4433(8) 12.9853(3) 90.1310(7) 5829.3(2) 4 1.352 -100 0.564 Rigaku R-AXIS RAPID Mo KR (λ = 0.71069 A˚) graphite 60 68 655 17 203 (Rint = 0.0246) 14 504 empirical 0.7164 (min.) 1.0000 (max.) 0.0343 0.0871 0.0422 0.0927 16 866/0/699 1.037 3.102 and -0.768

of 3-BF4 and 4-BPh4 were located on difference Fourier maps and refined isotropically. The details of crystal data and the results of the analyses are listed in Table 1.

Acknowledgment. The present work was supported by a Grant-in-Aid for Science Research on Priority Research (No. 18064007, Synergy of Elements) from the Ministry of Education, Culture, Sports, Science and Technology, Japan, and by a Grant-in-Aid for Science Research (S)

(No. 18105002) from the Japan Society for the Promotion of Science. Supporting Information Available: Infrared spectra of 2b-PF6 and its isotopomers for the assignment of Ru-H and H-H vibrations; T1 measurements of 2a-BPh4, 2b-BPh4, and 2b-PF6; 1H NMR spectrum of complex 2b-PF6; ESI-MS spectra of complexes 3-BF4 and 4-PF6; DFT calculations; and X-ray crystallographic data for 2a-BPh4, 3-BF4, and 4-BPh4 in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org.